Multi-family housing is a large market for quite living and noise is one of the most litigated issues in apartment and condo construction. Building codes call for specific performance levels for noise and ASTM has separated the noise into airborne and structural causes. For many years, the common method of controlling vibration noise in wood and concrete floor/ceiling assemblies has been to pour a floating substrate on top of a void creating device commonly referred to as an “entangled net”. The entangled net is 90% air with more rigid polymer filaments supporting a separation fabric layer that suspended the floating substrate from 0.125″ to 1.0″ above a wood subsurface creating a void space. The void limited touching of the two solid materials (subfloor and floating substrate) limited the energy transfer. The limited energy transfer consequently limited the ability of the vibration waves to pass through to the ceiling side of the construction. There are a number of products in the market today that have these characteristics and are successful in achieving code compliance. There are also many developers that are keenly interested in higher levels of performance than this system provides.
The ASTM codes have a one number designation that identifies performance over a broad frequency range from 100 Hz to 3150 Hz. Product performance over the range is measured and deviations from the standard established limit the success of the system's performance. It has been proven that a void space creating material like an entangled net significantly enhances performance of the assembly.
The vibration noise passes in two manners. One means is through the actual touching of the floating floor to the filaments and the filament contact with the subfloor (conduction of vibration waves). The second method is through the air pressure build up within the “entangled net” product, “convection”. Consequently, thicker entangled net products perform better since the air is allowed to move more freely through the void space and limits air pressure.
Each flooring assembly performs differently for noise, add or subtract a component like wallboard, change a structural element from a 2″×10″ joist to an open web, add a resilient channel and the sound performance will change, sometimes dramatically. Assemblies are tested for their ability to control airborne sound and rated with a Sound Transmission Classification (STC) number. The higher the number, the better the job the assembly does of controlling airborne sound. Vibration or structural sound is rated in a similar manner with an ASTM test resulting in an Impact Insulation Classification number. Again, the higher the number, the better. Additionally, every assembly has a frequency at which it resonates. Just like a tuning fork, a floor will amplify a noise at a given frequency. The goal of the assembly is to bring that frequency down to an inaudible level, the lower the better, although humans can't hear much below 50 Hz.